Should hooking the %pdb IPython magic be default?

Right now, when you import `pudb.ipython`, it hooks `ip.debugger` so that `%pdb` enables PuDB instead of ipdb. Should this be changed to not happen by default? If not, should we enable some way to disable it. I think right now if you have this in your IPython profile there is no way to get back to ipdb or pdb.

Start by reading the `pudb.ipython` integration and how it assigns `ip.debugger` for the `%pdb` magic. Decide whether the default hook should change or whether an opt-out is needed, including how users can return to `ipdb` or `pdb`. Done means the behavior and configuration choice are documented and covered by appropriate tests.

Written by the indexing model from the issue text.